1. Field of the Invention
The present invention relates to single acting, variable displacement fluid pressure vane pumps and motors, such as fuel and hydraulic control pumps and motors for aircraft use, component parts thereof and to a method for balancing fluid pressures.
Over the years, the standard of the commercial aviation gas turbine industry for main engine fuel pumps has been a single element, pressure-loaded, involute gear stage charged with a centrifugal boost stage. Such gear pumps are simple and extremely durable, although heavy and inefficient. However, such gear pumps are fixed displacement pumps which deliver uniform amounts of fluid, such as fuel, under all operating conditions. Certain operating conditions require different volumes of liquid, and it is desirable and/or necessary to vary the liquid supply, by means such as bypass systems which can cause overheating of the fuel or hydraulic fluid and which require heat transfer cooling components that add to the cost and the weight of the system.
2. State of the Art
Vane pumps and systems have been developed in order to overcome some of the deficiencies of gear pumps, and reference is made to the following U.S. Patents for their disclosures of several such pumps and systems: U.S. Pat. Nos. 4,247,263; 4,354,809; 4,529,361 and 4,711,619.
Vane pumps comprise a rotor element machined with slots supporting radially-movable vane elements, mounted within a cam member and manifold having fluid inlet and outlet ports in the cam surface through which the fluid is fed radially to the inlet areas or buckets of the rotor surface for compression and from the outlet areas or buckets of the rotor surface as pressurized fluid.
Vane pumps that are required to operate at high speeds and pressures preferably employ hydrostatically (pressure) balanced vanes for maintaining vane contact with the cam surface in seal arcs and for minimizing frictional wear. Such pumps may also include rounded vane tips to reduce vane-to-cam surface stresses. Examples of vane pumps having pressure-balanced vanes which are also adapted to provide undervane pumping, may be found in U.S. Pat. Nos. 3,711,227 and 4,354,809. The latter patent discloses a vane pump incorporating undervane pumping wherein the vanes are hydraulically balanced in not only the inlet and discharge areas but also in the seal arcs whereby the resultant pressure forces on a vane cannot displace it from engagement with a seal arc.
Variable displacement vane pumps are known which contain a swing cam element which is adjustable or pivotable, relative to the rotor element, in order to change the relative volumes of the inlet and outlet or discharge buckets and thereby vary the displacement capacity of the pump.
Among the disadvantages of known vane pumps are their lack of durability, susceptibility to wear, complexity of rotor and cam structures, necessity for end sealing plates to seal the ends of the rotor for the purpose of containing the pressurized fluid, and other essential elements which can provide vane pumps with variable metering properties not possessed by gear pumps but which detract from their durability or life span relative to the comparative durability and life spans of gear pumps. In conventional vane pumps the rotor is splined upon and driven by a central drive shaft having small diameter journal ends/which are not strong enough to withstand the opposed inlet and outlet hydraulic pressure forces generated during normal operation. This problem is overcome by forming such pumps as double-acting pumps having opposed inlet arcs and opposed outlet or discharge arcs which balance the forces exerted upon the journal ends, as disclosed by the prior art such as U.S. Pat. Nos. 4,354,809 and 4,529,361, for example.